Patent Documents 1 and 2 disclose conventional, movable armrests. Each of these armrests has a structure such that the armrest body can rotate against a seat frame, and such that a coil spring is used as a lock spring for locking or unlocking the rotation of the armrest body.
The armrest disclosed in Patent Document 1 has a two-shaft structure wherein a stationary shaft and a movable shaft are coaxially assembled together, with a lock spring wound around said shaft structure. The stationary shaft is fixed to a seat frame and, while the stationary shaft is being fixed to the seat frame, the stationary shaft serves as a shaft portion that fixes the lock spring. The movable shaft is fixed to the armrest body so as to rotate together with the armrest body, and the lock spring is wound in such a way that the lock spring extends from the stationary shaft so as to reach the movable shaft. When the diameter of the lock spring is reduced, the rotation of the armrest body is locked via the movable shaft, and when the diameter of the lock spring is increased, the movable shaft and the armrest body can rotate.
In the armrest of Patent Document 1, the angle of the armrest body is adjusted in a non-step manner by such a two-shaft structure. In order for this two-shaft structure to have sufficient strength to support the armrest (i.e., strength to support the weight of a human arm placed on the armrest), it is necessary for the lock spring to be wound a certain number of turns around the stationary shaft in addition to being wound a certain number of turns around the movable shaft. Because of the large number of times that the lock spring must be wound around the movable shaft and the stationary shaft, the height of the lock spring becomes high, and the thickness of the entire armrest becomes large. Due to such constraints on the height of the lock spring due because it must be wound a sufficient number of turns, there are limitations in making the armrest thinner.
The armrest disclosed in Patent Document 2 uses only a stationary shaft that supports the rotation of the armrest body. The armrest disclosed in Patent Document 2 has a one-shaft structure wherein the lock spring is wound around the stationary shaft so that the rotation of the armrest body is locked or unlocked. While the stationary-side hook of the lock spring is fixed to the armrest body, the free-side hook of the lock spring moves in the lock spring's diameter-enlarging direction or its diameter-reducing direction. When the free-side hook moves in the diameter-reducing direction, locking by the lock spring is effected, and when the free-side hook moves in the diameter-enlarging direction, locking by the lock spring is undone. In this one-shaft structure, it is not necessary for the stationary shaft to be coaxially assembled with a movable shaft, and therefore the lock spring does not need to be wound around a movable shaft. Thus, the height of the lock spring can be short, and the armrest can be thin.
[Patent Document 1] Japanese Published Patent Application No. 2004-147791
[Patent Document 2] Japanese Published Patent Application No. 2003-299547